Custom Search
|
|
|
||
 MIL-HDBK-419A
2.11.2.1 Electrode Resistance. The resistance of ground, R, of a single vertical electrode of length
in cm,
and radius a in cm, emplaced in homogeneous soil of resistivity
(ohm - cm) is found from:
(2-77)
This equation may be used to estimate the penetration depth of conductive salt solutions in the soil adjacent to
the treated backfill. Since the backfill is conductive, the electrode radius therefore is not just that of the
metallic electrode, but initially the diameter of the hole filled with treated backfill. This large composite
electrode is referred to as the effective electrode. For a constant ground temperature, any reduction in
electrode resistance of a frozen saturated soil with time should be related to an increase in effective electrode
diameter, presumably through salt movement. This increase can be determined by the soil resistivity
from
equation 2-77 using the resistance to ground of the test electrode and the effective electrode radius measured
at the time of installation. Periodically, after installation, the resistance to ground should be remeasured and
the effective electrode radius can be calculated using the following form of equation 2-77 and using the soil
resistivity calculated earlier:
(2-78)
2.11.2.2 Installation and Measurement Methods.
2.11.2.2.1 Electrode Installation. Holes can be drilled with augers designed for use in frozen ground with hole
diameters ranging from 3.8 cm (1-1/2 in.) to 91.4 cm (3 ft) and depths seldom greater than 2 m (6 ft).
Hand-
held equipment, consisting of an electric drive or a 5-hp gasoline-powered drill can also be used for most of the
shallow, smaller-diameter holes. Both units could be used with a coring auger to drill holes up to 11 cm (4 in.)
in diameter in fine-grained frozen soils. A truck-mounted auger can be used for the larger-diameter vertical
holes drilled in coarse-grained materials. The horizontal electrodes can be hand-pushed and then driven into the
thin seasonally thawed layer.
Military 6.8 kg (M2A3) shaped charges (used only by qualified personnel) can also be employed to produce
vertical holes. Their similar performances in a range of frozen materials, with penetration approaching the
length of standard electrodes, make this charge size ideal for electrode installation. The volume of several of
the drilled holes can also be expanded by using C-4 block explosives.
2.11.2.2.2 Backfill. Reduction of contact potential is important in establishing a good electrical ground. In
frozen soil, ice can form around the electrode, causing high contact resistance. Ice formation on the rod
surface is likely since the rod is easily chilled by exposure of the upper end to low air temperatures. The
beneficial effect of pouring untreated water around an electrode will only be short-term in cold environments.
Therefore, the use of conductive backfill with a low freezing point becomes paramount to attain good ground or
earth contact.
2-71
|
 
|
|
|
||
